Thermoplastic resin film and a method for producing the same

ABSTRACT

A thermoplastic resin film at least oriented in the transverse direction with minimized bowing phenomenon is provided. The thermal shrinkage stress in the transverse direction of the film satisfies the following formula I, and the thermal shrinkage factor of said film in the transverse direction at a temperature that is 40° C. higher than the glass transition temperature of said resin is 5% or less: 
     
         (σ.sub.2 /σ.sub.1)≦ 1.0                 (I) 
    
     wherein σ 1  is the thermal shrinkage stress (kg/mm 2 ) of the film in the transverse direction at a temperature that is 70° C. higher than the glass transition temperature of said resin, and σ 2  is the thermal shrinkage stress (kg/mm 2 ) of the said film in the transverse direction at a temperature that is 80° C. lower than the melting temperature of said resin.

This is a continuation of application Ser. No. 07/890,366 filed on May26, 1992, now abandoned, which is, in turn, a continuation of Ser. No.07/584,465 filed on Sep. 18, 1990, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermoplastic resin film possessinguniform physical and chemical properties in the transverse direction ofthe film. Furthermore, the present invention relates to a method forproducing a thermoplastic resin film possessing uniform physical andchemical properties in the transverse direction by suppressing thebowing phenomenon which occurs in the process of transverse drawing.

2. Description of the Prior Art

Thermoplastic resin films, particularly, biaxially oriented polyesterfilms, polyamide films, polyolefin films, polyvinyl resin films,polyphenylene-sulfide films, etc., are used for packing as well as invarious industrial and other applications.

In conventional methods for the manufacture of biaxially oriented films,the physical properties in the transverse (lateral) direction of theobtained films are generally nonuniform. This nonuniformity of filmsarises with particular frequency in transverse drawing processes. Intransverse drawing processes, the film is drawn by holding both sidemargins of the film in a tenter with a clasping means such as clips andimparting tension in the transverse direction by successively shiftingthe clasping means. Ordinarily, this drawing process is followed byheat-setting, thereby obtaining a desired transversely drawn film. Inthis process, the side margins of the film are securely constrained bythe clasping means, but in the central portion of the film, the effectsof the clasping means are relatively small and the constraining force isaccordingly weak. Consequently, when a long film is subjected to atransverse drawing by passing through a tenter, the central portion ofthe film is affected by the stress in the longitudinal directiongenerated by the transverse drawing and longitudinal movement of thefilm, or affected by the contraction stress generated by theheat-setting process. For example, in a case where drawing andheat-setting are performed consecutively in the same tenter, if straightlines are drawn on the surface of the film in the direction prior toentering the tenter, then these straight lines are first deformed, inthe area where the drawing process commences, into a convex shape whichis convex toward the direction of advance of the film, then, in the areaimmediately preceding the completion of the drawing process, thedeformed lines are restored to recover their shape, and immediatelyafter the completion of drawing, the lines are then deformed into aconcave shape. Furthermore, at the beginning of an area of theheat-setting process, this concave deformation reaches a maximum, andthereafter these curves pass through the tenter without undergoing anyfurther deformation, hence, the concave deformation remains after thefilm has emerged from the tenter. This phenomenon is known as bowing.This bowing phenomenon is the cause of the nonuniformity of physicalproperties in the transverse direction of the film.

Owing to the bowing phenomenon, the principal orientation axes near thetwo side margins of the film deviate toward the longitudinal axisconsiderably. That is, the angles of the orientation axes at the centraland side marginal portions of the film tend to differ. Consequently, forexample, the physical constants with respect to the longitudinaldirection, such as the thermal shrinkage factor, thermal expansionfactor, wet swelling factor, etc., are different in the central and inthe side marginal portions of the film. In the applications of suchfilms to packing or wrapping, various problems arise, e.g., the printingpitch deviates in processes such as printing, lamination or bag-making;blots appear on the surface of the film; the film curls or winds, etc.Moreover, as regards industrial applications, for example, when suchfilms are used as the base films for floppy disks, anisotropy occurswithin the surface and consequently various problems arise, such asdeterioration of magnetic recording characteristics, etc.

Japanese Patent Publication No. 35-11774 discloses a method for thetransverse drawing of thermoplastic resin films. The method comprises arelaxation process (i.e., substantially a cooling process) in thetemperature range of 20° C. to 150° C. that is interposed between thetransverse drawing and heat-setting processes. However, if this methodis employed, the bowing phenomenon is still not reduced adequately.Japanese Laid-Open Patent Publication No. 50-73978 discloses a methodfor producing a stretched thermoplastic film for reducing the bowingphenomenon. In this method, a process for passing a film through a groupof nip rolls is interposed between the drawing and heat-settingprocesses of the film. This Publication discloses that the temperatureof this intermediate zone where the nip rolls are disposed should beequal to, or higher than the glass transition temperature of thethermoplastic resin film. However, at the point where the film comesinto contact with the nip rolls (i.e., nip point), the rigidity of thefilm is low, therefore improvement with respect to the bowing phenomenonis not still adequate. Furthermore, in Japanese Patent Publication No.63-24459, a method is proposed in which a film passes through a nip rollthat is located in a middle portion of the film after the drawingprocess of the film, while the two side margins of the film are held sothat only the central portion of the film is forcibly advanced. However,in this method, the nip roll is located in a high-temperature areawithin the tenter, and the roll and its peripheral devices must becooled. Since the film temperature is high, the roll may damage thefilm, hence, the range of applicability of this method is restricted.Japanese Patent Publication No. 62-43856 discloses a method in which afilm immediately after drawing is cooled to a temperature equal to orlower than the glass transition temperature of the film, and thenmultistage heat-setting is applied, while the drawing in the transversedirection is performed simultaneously with the heat-setting. However,this method comprises a complex array of processes including, inaddition to a cooling process, a multistage heat-setting process and are-drawing process, and the stable regulation of the temperature withinthe tenter and film temperature over a long period of time is difficult.Japanese Laid-Open Patent Publication No. 62-183327 discloses a methodin which longitudinal drawing of a film is performed, followed bytransverse drawing in the tenter, and then heat-setting. In thisprocedure, the provision of a preheating zone between a transversedrawing zone and a heat-setting zone is proposed, wherein only themarginal portions of the film are preheated to a temperature equal to orhigher than the glass transition temperature and equal to or lower thanthe heat-setting temperature. However, in this method, the temperatureof the preheating zone must be regulated while maintaining a temperaturegradient in the transverse direction, and therefore the regulation ofthe film temperature over a long period of time is difficult. JapaneseLaid-Open Patent Publication No. 1-165423 discloses a method, in which afilm, after transverse drawing, is cooled to a temperature equal to orlower than the transverse drawing temperature, and the film is redrawnin the transverse direction while raising the temperature in severalstages. However, this method, like the method of the above-mentionedJapanese Patent Publication No. 62-43856, comprises a complex array ofprocesses including, in addition to a cooling process, a multistageheat-setting process and a redrawing process, and the stable regulationof the temperature and film temperature within the tenter over a longperiod of time is difficult. Moreover, this patent Publication disclosesthat the length of the cooling zone should desirably be at least 1/2 thewidth of the film, but the reasons for this are not disclosed. JapanesePatent Publication Nos. 1-25694 and 1-25696 disclose a method, in whichthe direction of travel of a film is reversed at specified times whentransverse drawing and heat-setting are performed. However, in thismethod, in order to reverse the direction of travel of the film, thefilm must be coiled onto a reel at a predetermined time during theprocess, and, since this is an off-line manufacturing method, the methodinvolves various problems, such as limitations with respect toproductivity, etc.

Thus, various procedures intended to reduce bowing have been proposed,but all of these proposals are concerned with manufacturing processesand apparatus, and hitherto no invention for this purpose has beenrealized through consideration of the characteristics of the filmitself, such as molecular orientation, etc. In order to measure thedegree of bowing on the basis of molecular orientation angles, asmentioned in Japanese Laid-Open Patent Publication Nos.58-215318 and61-8326, the molecular orientation angle must be measured along theentire width of the film in order to determine the degree of bowing.This is due to the fact that, in the central portion of the film, thereis almost no deviation in the direction of the principal axis ofmolecular orientation, irrespective of the degree of bowing, thereforethe assessment of the degree of bowing from the measurement of physicalcharacteristics at an arbitrary location of the film is impossible.

SUMMARY OF THE INVENTION

The thermoplastic resin film of this invention, which overcomes theabove-discussed and numerous other disadvantages and deficiencies of theprior art, is a thermoplastic resin film at least oriented in thetransverse direction, wherein the thermal shrinkage stress in thetransverse direction of the film satisfies the following formula I, andthe thermal shrinkage factor of said film in the transverse direction ata temperature that is 40° C. higher than the glass transitiontemperature of the resin is 5% or less:

    (σ.sub.2 /σ.sub.1)≦1.0                  (I)

wherein σ₁ is the thermal shrinkage stress (kg/mm²) of the film in thetransverse direction at a temperature that is 70° C. higher than theglass transition temperature of the resin, and σ₂ is the thermalshrinkage stress (kg/mm²) of the film in the transverse direction at atemperature that is 80° C. lower than the melting temperature of theresin.

The thermoplastic resin film of this invention is a thermoplastic resinfilm at least oriented in the transverse direction, wherein themolecular orientation angle in a marginal portion of the film, measuredby microwave techniques, satisfies one of the following formulae II,III, and IV:

    -90°<A≦-60°                           (II)

    -30°≦A≦30°                     (III)

    60°≦A≦90°                      (IV)

wherein A is a molecular orientation angle (°), said angle beingmeasured with the longitudinal axis corresponding to 0° and clockwiserotation being regarded as positive.

The thermoplastic resin film of this invention is a thermoplastic resinfilm at least oriented in the transverse direction, wherein thedifference between the molecular orientation angles at any different twopoints on a straight line in the transverse direction measured bymicrowave techniques satisfies the following formula V:

    ΔΘ.sub.or ×w/wf≦64.0              (V)

wherein ΔΘ_(or) is the difference of the molecular orientation angles(°) measured at the said two points, Wf is the distance (m) of the saidpoints, and W is the width (m) of the film.

In a preferred embodiment, the thermoplastic resin is at least oneselected from the group consisting of polyesters, polyamides, andpolypropylenes.

The method for producing a thermoplastic resin film at least oriented inthe transverse direction of this invention comprises the steps ofdrawing a thermoplastic resin film in the transverse direction in adrawing zone, cooling the said film to a temperature equal to or lowerthan the drawing temperature in a cooling zone, and heat-setting thesaid film in a heat-setting zone, wherein the length of the cooling zonesatisfies the following formula VI:

    (L/W)≧1.0                                           (VI)

wherein L is the length (m) of the cooling zone, and W is the width (m)of the film after the drawing was carried out.

The method for producing a thermoplastic resin film of this inventioncomprises the steps of, drawing a thermoplastic resin film in thetransverse direction in a drawing zone, cooling said film to atemperature equal to or lower than the glass transition temperature ofthe resin in a cooling zone, and heat-setting the said film in aheat-setting zone,

wherein the said cooling zone is provided with a group of nip rolls, andthe length of the cooling zone satisfies the following formula VII:

    (L/W)≧0.25 (2.0-W.sub.N /W).sup.2                   (VII)

wherein L is the length (m) of the cooling zone, W is the width (m) ofthe film, and W_(N) (m) is the width (m) of the widest nip roll in thegroup of nip rolls.

In a preferred embodiment, the thermoplastic resin used for theabove-mentioned thermoplastic resin material film is at least oneselected from the group consisting of polyesters, polyamides, andpolypropylenes.

In a preferred embodiment, the thermoplastic resin film of thisinvention is a biaxially oriented polyamide film, wherein the boilingwater shrinkage distortion factors (%) at any different two points on astraight line in the transverse direction, and the molecular orientationangles at any different two points on a straight line in the transversedirection measured by microwave techniques satisfy the following formulaVIII:

    ΔBS×ΔΘ.sub.or /Wf.sup.2 ≦44.0 (VIII)

wherein ΔBS is, in terms of absolute value, the difference between saidboiling water shrinkage distortion factors (%) measured at the said twopoints; ΔΘ_(or) is the difference between the molecular orientationangles measured at the said two points, and Wf is the distance (m)between the points of measurement of the molecular orientation angle,

said boiling water shrinkage distortion factor at each of the saidpoints being the value obtained by measuring the boiling water shrinkagefactors (%) at +45° and -45° from the longitudinal axis of the filmclockwise rotation around the longitudinal axis being as positive, andsubtracting said boiling water shrinkage factor in the -45° directionfrom the said factor in the +45° direction.

A method for producing a thermoplastic resin film of this inventioncomprises the steps of drawing a thermoplastic resin film in thetransverse direction, and heat-setting said film,

wherein said heat-setting process comprises at least one of theprocesses for shrinking said film in the transverse direction, and forblowing water vapor at a temperature of 95° C. or more.

The method for producing a thermoplastic resin film of this inventioncomprises the steps of, drawing a thermoplastic resin film in thelongitudinal direction in a first drawing zone, drawing the said film inthe transverse direction in a second drawing zone, and cooling the saidbiaxially drawn film at a temperature lower than the temperature of saiddrawing steps in a cooling zone,

wherein the length of said cooling zone satisfies the following formulaVI:

    (L/W)≧1.0                                           (VI)

wherein L is the length (m) of said cooling zone, and W is the width (m)of said film after the drawing in the transverse direction was carriedout.

Thus, the invention described herein makes possible the objectives of:

(1) providing a thermoplastic resin film possessing uniform physicalproperties (including mechanical properties) and chemical properties inthe transverse direction;

(2) providing a thermoplastic resin film possessing the aforesaiddesirable characteristics and is highly suitable for use as wrapping orpacking film; and

(3) providing a method for producing a thermoplastic resin filmpossessing uniform physical and chemical properties in the transversedirection by suppressing the bowing phenomenon which occurs in thedrawing process.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be better understood and its numerous objects andadvantages will become apparent to those skilled in the art by referenceto the accompanying drawings as follows:

FIG. 1 shows the definition of molecular orientation angle of thethermoplastic resin film in one embodiment of the present invention.

FIG. 2 shows an example of the apparatus for producing thermoplasticresin films of the present invention.

FIG. 3 shows a bowing distortion which occurs in the process ofproducing a stretched thermoplastic film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, the present invention will be explained indetail.

Drawn or stretched thermoplastic resin film oriented at least in thetransverse direction, as referred to in the description of the presentinvention, are films which have been drawn at least 2.5 times in thetransverse direction, thereby imparting a molecular orientation. Morespecifically, these may be either biaxially oriented films obtained bytransverse drawing of longitudinally drawn films (i.e., films which havepreviously been oriented in the lengthwise direction), or transverseuniaxially drawn films obtained by transverse orientation of essentiallyunoriented films. Alternatively, the said films may also be biaxiallyoriented films obtained by longitudinal drawing of transverse uniaxiallyoriented films. After drawing at least in the transverse direction, theaforesaid oriented films should desirably be heat treated at atemperature in the range of from the above-mentioned drawing temperatureto a temperature 20° C. lower than the melting temperature of the saidthermoplastic resin. In the present specification, the term "transversedirection of the film" refers to the direction perpendicular to thedirection of travel of the film during the film manufacturing process.

In the thermoplastic resin films of the present invention, the thermalshrinkage stress satisfies the following formula I, and the thermalshrinkage factor in the transverse direction (HS_(TD)) is 5% or less:

    (σ.sub.2 /σ.sub.1)≦1.0                  (I)

wherein σ₁ is the thermal shrinkage stress (kg/mm²) of the film in thetransverse direction at a temperature that is 70° C. higher than theglass transition temperature of the thermoplastic resin, and σ₂ is thethermal shrinkage stress (kg/mm²) of the film in the transversedirection at a temperature that is 80° C. lower than the meltingtemperature of the resin. The thermal shrinkage stresses σ₁ and σ₂ weremeasured by means of a Thermo-mechanical Analyzer (TM-3000),manufactured by the Shinku-Riko Inc. The thermal shrinkage factor in thetransverse direction (HS_(TD)) of the film is the value (%) of theshrinkage in the transverse direction when the film has been maintainedfor 30 minutes at a temperature that is 40° C. higher than the glasstransition temperature of the resin. The ratio σ₂ /σ₁ should desirablynot exceed 0.9. If the ratio σ₂ /σ₁ exceeds 1.0, then the distortion ofthe film resulting from bowing will be large.

In general, the physical properties of the film are determined not onlyby the crystalline but also the non-crystalline portions of the film. Inparticular, the thermal shrinkage behavior of resin films is said to belargely governed by the non-crystalline portion. In the presentinvention, molecular orientation angles were measured with an apparatusfor assessing the orientation of non-crystalline chains by means ofmicrowaves, in order to determine the state of molecular orientation ofresin films. As the apparatus, molecular orientation analyzer MOA-2,manufactured by the Kanzaki Paper Company, Ltd. was used. In a sidemarginal portion of the thermoplastic resin films of the presentinvention, the molecular orientation angle as measured by microwavetechniques satisfies one of the following formulae II, III, and IV.

    -90°<A≦-60°                           (II)

    -30°≦A≦30°                     (III)

    60°≦A≦90°                      (IV)

wherein A is a molecular orientation angle (°), the said angle beingmeasured with the longitudinal axis corresponding to 0° and clockwiserotation being regarded as positive. That is, the angles A satisfyingone of the above formulae II, III and IV are those lying in the sectorsindicated by oblique shading in FIG. 1. A transversely drawn filmincluded in the present invention satisfies one of the above-mentionedformulae II and IV. A biaxially drawn film included in the presentinvention satisfies one of the above-mentioned formulae II, III and IV.The side marginal portion of the thermoplastic resin film is defined asan area which occupies one-tenth of the width from one end toward themiddle.

Also, in the thermoplastic resin films of the present invention, thedifference between the molecular orientation angles at any different twopoints on a straight line in the transverse direction measured bymicrowave techniques satisfies the following formula V:

    ΔΘ.sub.or ×w/wf≦64.0              (V)

wherein ΔΘ_(or) is the difference of the molecular orientation angles(°) measured at the said two points, Wf is the distance (m) of the saidpoints, and W is the width (m) of the said film.

The molecular orientation angle changes in an almost linear fashion fromthe central to the side marginal portion of the film. For example, whenthe molecular orientation angle in the central portion of the film isalong the transverse direction, the molecular orientation angle deviatestoward the longitudinal direction as the point of measurement approachesthe side edge of the film. If the molecular orientation angles satisfythe foregoing condition, then the change of the molecular orientationangle over the entire width of the film is small, therefore the film canbe regarded as displaying fairly uniform physical properties.Conversely, if the molecular orientation angles do not satisfy theforegoing condition, then the major orientation axis appreciablyinclines locally, and anisotropy of physical properties arises. Forexample, assessment of the curling observed when the film was formedinto a bag shape, regarded as an index of anisotropy, revealed that bagswith relatively slight curl along the entire width of the film wereobtained if films satisfying formula V were used.

The thermoplastic resins appropriate for use in the present inventioninclude, for example, polyester resins such as polyethyleneterephthalate, polyethylene 2,6-naphthalate, polyethylene isophthalate,polybutylene terephthalate, etc.; polyamide resins such as Nylon 6,Nylon 6-6, etc.; polyolefin resins such as polypropylene, polyethylene,etc.; polyphenylene sulfides; polyether sulfones; polysulfones;polyetheretherketones; polyetherketoneketones; andpolyethylenetrimellitateimide. Many other homopolymers, copolymers,polymer mixtures, or polymer complexes can be used. In particular,polyester resins, polyamide resins and polypropylene are especiallyappropriate for the present purpose.

In cases where polyamide films are used as the aforesaid thermoplasticresin films, the following conditions should be met. Namely the boilingwater shrinkage distortion factors (%) at any different two points on astraight line in the transverse direction, and the molecular orientationangles at any different two points on a straight line in the transversedirection measured by microwave techniques satisfy the following formulaVIII:

    ΔBS×ΔΘ.sub.or /Wf.sup.2 ≦44.0 (VIII)

wherein ΔBS is, terms of absolute value, the difference between the saidboiling water shrinkage distortion factors (%) measured at the said twopoints; ΔΘ_(or) is, in terms of absolute value, the difference betweenthe molecular orientation angles measured at the said two points, and Wfis the distance (m) between the points of measurement of the molecularorientation angle.

The boiling water shrinkage distortion factor at each of said points isthe value obtained by measuring the boiling water shrinkage factors (%)at +45° and -45° from the longitudinal axis of the film (clockwise)rotation around the longitudinal axis being taken as a positive numberof degrees, and subtracting said boiling water shrinkage factor in the-45° direction from said factor in the +45° direction.

The boiling water shrinkage factor is determined by the followingmethod. A first and second lines are drawn through a given point on asample film in the +45° and -45° direction, respectively. Next, thesample film is seasoned for 2 hours at standard conditions (i.e., 23°C., 50% RH). Then, the length of each reference line is measured anddenoted by l₀ and l_(0'), respectively. The sample film is immersed inboiling water (100° C.) for 30 minutes, and then subjected to seasoningat the standard conditions for 30 minutes. The length of each of thereference lines is measured, and denoted by l₁ and l_(1'), respectively.The boiling water shrinkage factors in the +45° direction and -45°direction are calculated as follows, respectively: ##EQU1##

The thermoplastic resin films of the present invention are manufacturedby the following method. The method comprises the steps of heating athermoplastic resin at a temperature that is higher than the meltingtemperature of the resin, thereby fusing the resin; extruding the resinin a film form from an extrusion device including a slit die onto acooling drum; drawing the film in the longitudinal direction by means ofa group of rollers with controllable speed; drawing the film in thetransverse direction in a tenter, and, if necessary, subjecting the filmto heat-setting. Finally, the film is reeled up with a device such as afilm winder. In the process of the present invention, the conditions forfilm formation and drawing, i.e., the conditions of melting andextrusion of the resin, casting conditions, longitudinal drawingconditions, transverse drawing conditions, heat-setting conditions andwinding conditions, etc., can be selected in a manner appropriate forthe realization of the desired film characteristics.

In a preferred embodiment of the present invention, the thermoplasticresin film of this invention can be produced by the following method.The method comprises the step of a cooling process which is interposedbetween the transverse drawing and heat-setting processes. In thecooling step, the temperature of the film is lowered to a temperatureequal to or lower than the drawing temperature, and the length of thecooling zone satisfies the following formula VI:

    (L/W)≧1.0                                           (VI)

wherein L is the length (m) of the cooling zone and W is the width (m)of the film after the transverse drawing was carried out. The length Lof the cooling zone is a length that is set so that the temperature ofthe cooling zone is lower than that of the drawing zone, and the widthof the drawn film passing through said cooling zone is 90% or more ofthe film before passing through said cooling zone. The width W of thefilm means the distance between a pair of tenter clips at the outlet ofthe tenter.

The value of L/W which is the ratio of the length L of the cooling zoneto the film width W does not essentially depend upon the running speedof the film in the tenter. However, if the running speed increases, thefilm would move a longer distance until the film is cooled sufficiently.Hence, if the running speed of the film in the tenter is increased, thenthe ratio L/W should also be increased accordingly. For example, if thespeed is increased by a factor of 2, then the value of the ratio L/Wshould desirably be increased to 1.5 times the value prior to the speedincrease.

The larger the value of the ratio L/W is, the greater the efficacy indiminishing bowing phenomenon, and in fact, the length L of the coolingzone should desirably be selected so that L/W≧2.0, or, more preferably,L/W≧3.0.

The length L of the aforesaid cooling zone required in order to avoidthe bowing phenomenon was determined by establishing a numerical modelto which the finite element method could be applied, and estimating thedrawing stress propagated from the marginal portion to the centralportion of the film by numerical analysis. The values so calculatedrevealed that when the ratio L/W is 1.0, then the propagated drawingstress is approximately 1/2, compared with a case that the coolingprocess was not employed. If L/W is 2.0, then the propagated stress isapproximately 1/10, and if L/W is 3.0, then the propagated stress isnearly zero. These numerical results were confirmed by experiments,which demonstrated that the numerically calculated estimates wereappropriate in all cases.

As for the temperature of this cooling process, adequate efficacy isobtained when the temperature is lower than the drawing temperature. Thelower the cooling temperature is, the greater the effect in reducing thedegree of bowing, and in fact the selection of a cooling temperaturelower than the glass transition temperature is preferable.

Furthermore, in the aforesaid cooling process, the reduction of bowingeffects is even greater if the film is cooled without holding both sidemarginal portions. For example, the transverse drawing and heat-settingprocesses are conducted in separate tenters, and the film is cooled byallowing the said film to travel through the ambient atmosphere betweenthe transverse drawing and heat-setting processes. In this case, if thelength of the cooling zone is chosen so that L/W≧1.0, then excellentthermoplastic resin films with minimized bowing phenomenon can beobtained.

In another preferred embodiment of the present invention, the filmshould desirably be passed through a group of nip rolls withcontrollable speed, either during the cooling process or after theheat-setting process, or both. By the process mentioned above, thebowing phenomenon can be reduced effectively. The group of nip rollsshould desirably be a combination a roll with metal mirror surfaces anda roll with rubber elastomer surfaces. Moreover, easy speed control ofthe rolls is necessary, since a difference between the speed of the niprolls and that of the tenter clips may cause a tension in the film.Moreover, when a pair of nip rolls is employed, the speed of either oneor both of each pair of nip rolls should desirably be controllable.

The cooling process should desirably be performed in a cooling zoneequipped with a group of nip rolls, the length L of said cooling zonesatisfying the following formula VII:

    (L/W)≧0.25 (2.0-W.sub.N /W).sup.2                   (VII)

wherein L is the length (m) of the cooling zone, W is the width (m) ofthe film after the transverse drawing, and W_(N) is the width (m) of thewidest nip roll in the group of nip rolls. The greater the width W_(N),the more pronounced the effect in reduction of bowing, and the ratioW_(N) /W of the width W_(N) of the widest nip roll to the width W of thefilm should desirably be at least 0.2. If this ratio is less than 0.2,then reduction of bowing phenomenon is insufficient, and also shearingforce is generated by the nip rolls, wrinkles occur, and productivitycharacteristics deteriorate. Either one pair or several pairs of niprolls in common use may be employed as the nip rolls required for thepresent invention. Alternatively, a roll group composed of a combinationof nip rolls and other types of rolls may be used for the presentpurpose. Also, a special type of roll described in Japanese PatentPublication No. 60-255584 may be used either singly or assembled into agroup, or this special type of roll may be used in a group combined withnip rolls or other suitable types of rolls. The width W_(N) of thewidest nip roll refers to the width of the roll that comes into contactwith the film. The length L of the cooling zone, the width W of the filmand the width W_(N) of the widest nip roll are to be expressed in thesame units, ordinarily meters (m).

According to the method of the present invention, the transverse drawingprocess is executed in two or more drawing sections, and desirably, thetemperature of the drawing sections are set progressively higher towardthe cooling zone. Furthermore, the heat-setting process desirablycomprises a first and second heat-setting processes. The firstheat-setting process is carried out at a temperature in the range of 50°C. higher than the glass transition temperature to 20° C. lower than themelting temperature, while shrinking the said film by 0 to 10% in thetransverse direction, and said second heat-setting process is carriedout at a temperature in the range of 100° C. higher than said glasstransition temperature to 20° C. lower than the melting temperature,while shrinking the said film by 0 to 10% in the longitudinal direction.The first heat-setting step is carried out by narrowing the width of theclips of the tenter by 0-10%, while the second heat-setting step iscarried out by using a method such as passing the film through a niproll with controllable speed in order to shrink the length of the filmby 0-10%.

The following conditions are desirable when the thermoplastic resin filmof the present invention is produced using a polyamide resin. Undrawn,uniaxially oriented or biaxially oriented polyamide films containing atleast 1.0% by weight of water is used as a material film. The polyamidefilm should initially be drawn in the longitudinal direction, then drawnin the transverse direction, and then subjected to the aforesaid coolingprocess. In the heat-setting process, biaxially oriented polyamide filmsshould be subjected to heat-setting while undergoing shrinkage in thelongitudinal direction, or while undergoing heat treatment in steam at atemperature of at least 95° C. These two types of processing may also beapplied simultaneously during the heat-setting process.

In the following, the present invention will be described in detail withreference to non-limiting examples.

EXAMPLES

In the Examples and Comparative examples described below, thermoplasticresin films can be produced by using, for example, an apparatus shown inFIG. 2. First, a thermoplastic resin is extruded from a T-die 1, thenabruptly cooled by the chill roll 2, thus forming a film 20. This film20 is drawn in the longitudinal direction by the roll drawing machine 3,next, the side margins of the film 20 are gripped by clips 5 of thetenter 4, then the film 20 passes through a tenter 4 which comprises apreheating zone 6, transverse drawing zone 7, cooling zone 8, andheat-setting zones 9 and 10, as follows. First, the film passes throughthe preheating zone 6 and undergoes preliminary heat treatment, and isthen transversely drawn in the transverse drawing zone 7. Next, the film20 is cooled in the cooling zone 8 and then subjected to heat-setting inthe two heat-setting zones 9 and 10. After this, the clip 5 is detached,then the film 20 emerges from the tenter and is taken up onto winder 11.

In the following Examples and Comparative examples, the bowingdistortion was measured as follows. First, a line traversing the entirefilm width was inscribed on the film surface prior to entering thetenter 4. As shown in FIG. 3, the line on the finally obtained film wasdeformed into bow-shaped curves, the bowing distortion B was calculatedfrom the following equation:

    B(%)=b/w×100

wherein, W is the width (mm) of the film, and b is the maximumindentation (mm) of the bowing curve.

Example 1

Polyethylene terephthalate was melted and extruded from a T-die, andafter forming into a film on a chill roll, drawn 3.6 times in thelongitudinal direction with a roll drawing machine. Then the film wasdrawn 3.7 times in the transverse direction, and finally subjected toheat-setting treatment in a tenter, thereby obtaining a biaxiallyoriented polyethylene terephthalate film. The respective temperatureswithin the tenter were 90° C. for preheating, 100° C. for drawing, 55°C. for the subsequent cooling, and 220° C. for heat-setting. Afterfurther heat treatment at 200° C., the film was cooled to 100° C.,removed from clips and then wound in the usual manner. In the presentcase, the ratio L/W of the length L of the cooling zone and the width Wof the film was 1.0.

Example 2

A biaxially oriented polyethylene terephthalate film was obtained by thesame procedure as in Example 1, except that in the present case theratio L/W was 3.0.

Example 3

Polyethylene terephthalate was melted and extruded from a T-die, andafter forming into a film on a chill roll, drawn 3.7 times in thetransverse direction at 100° C. in a tenter. Then the film was drawn 3.6times in the longitudinal direction with a roll drawing machine. Thefilm was subjected to heat-setting treatment at 220° C. and further heattreatment at 200° C., and then cooled to 100° C. in another tenter,thereby obtaining a biaxially oriented polyethylene terephthalate film.Then the film was removed from clips and wound in the usual manner. Inthe present case, there was a substantial cooling process in which thefilm was cooled to a temperature of 65° C. or lower between thetransverse drawing process and the heat-setting process, and the ratioL/W was 5.0 or more.

Comparative Example 1

A biaxially oriented polyethylene terephthalate film was obtained by thesame procedure as in Example 1, except that in the present case therewas no cooling process (i.e., L/W=0).

Comparative Example 2

A biaxially oriented polyethylene terephthalate film was obtained by thesame procedure as in Example 1, except that in the present case theratio L/W was 0.5.

Example 4

A nylon 6 resin was melted and extruded from a T-die, and after forminginto a film on a chill roll, drawn 3.25 times in the longitudinaldirection with a roll drawing machine. Then the film was drawn 3.5 timesin the transverse direction, and finally subjected to heat-settingtreatment in a tenter, thereby obtaining a biaxially oriented nylon 6film. The respective temperatures within the tenter were 60° C. forpreheating, 85° C. for drawing, 40° C. for the subsequent cooling, and235° C. for heat-setting. After further heat treatment at 210° C., thefilm was cooled to 100° C., removed from clips and then wound in theusual manner. In the present case, the ratio L/W was 3.0.

Comparative Example 3

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 4, except that in the present case there was no coolingprocess (i.e., L/W=0).

Table 1 shows the ratio (σ₂ /σ₁) of thermal shrinkage stresses, thethermal shrinkage factor (HS_(TD)) in the transverse direction at atemperature that is 40° C. higher than the glass transition temperatureof the resin, and the bowing distortion (B), of the films obtained inthe above-mentioned Examples and Comparative Examples. The ratios (σ₂/σ₁) of thermal shrinkage stresses and the thermal shrinkagefactors(HS_(TD)) in the transverse direction in this table are thosemeasured in the central portion of each film.

                  TABLE 1                                                         ______________________________________                                               Resin                          Bowing                                         used for                 HS.sub.TD                                                                           distortion                                     film     L/W     σ.sub.2 /σ.sub.1                                                          (%)   B (%)                                   ______________________________________                                        Example 1                                                                              Polyethylene                                                                             1.0     0.85  1.2   5.7                                            terephthalate                                                        Example 2                                                                              Polyethylene                                                                             3.0     0.66  2.0   2.8                                            terephthalate                                                        Example 3                                                                              Polyethylene                                                                             5.0     0.73  0.8   2.6                                            terephthalate                                                        Comparative                                                                            Polyethylene                                                                             0.0     1.23  1.2   7.8                                   Example 1                                                                              terephthalate                                                        Comparative                                                                            Polyethylene                                                                             0.5     1.02  1.0   7.2                                   Example 2                                                                              terephthalate                                                        Example 4                                                                              Nylon 6    3.0     0.83  3.8   4.5                                   Comparative                                                                            Nylon 6    0.0     1.29  2.2   9.4                                   Example 3                                                                     ______________________________________                                    

As shown in Table 1, the values of the ratio (σ₂ /σ₁) were greater than1.0 in the films obtained in Comparative Examples 1 and 2, indicating apronounced degree of bowing. On the other hand, thermoplastic resinfilms of the present invention obtained in Examples 1-4 displayed onlyslight bowing distortion, indicating highly uniform physical propertiesin the transverse direction.

Example 5

A polyethylene terephthalate resin was melted and extruded from a T-die,and after forming into a film on a chill roll, drawn 3.5 times in thelongitudinal direction with a roll drawing machine. Then the film wasdrawn 3.6 times in the transverse direction, and finally subjected toheat-setting treatment in a tenter, thereby obtaining a biaxiallyoriented polyethylene terephthalate film. The respective temperatureswithin the tenter were 90° C. for preheating, 100° C. for drawing, 40°C. for the subsequent cooling, and 210° C. for heat-setting. The filmwas then wound in the usual manner. In the present case, the ratio L/Wwas 1.0.

Example 6

A biaxially oriented polyethylene terephthalate film was obtained by thesame procedure as in Example 5, except that in the present case theratio L/W was 2.0.

Example 7

A biaxially oriented polyethylene terephthalate film was obtained by thesame procedure as in L/W was 3.0.

Example 8

A biaxially oriented polyethylene terephthalate film was obtained by thesame procedure as in Example 5, except that in the present case thetemperature of the cooling zone was 65° C.

Comparative Example 4

A biaxially oriented polyethylene terephthalate film was obtained by thesame procedure as in Example 5, except that in the present case therewas no cooling process (i.e., L/W=0).

Example 9

A nylon 6 resin was melted and extruded from a T-die, and after forminginto a film on a chill roll, drawn 3.3 times in the longitudinaldirection with a roll drawing machine. Then the film was drawn 3.4 timesin the transverse direction, and finally subjected to heat-settingtreatment in a tenter, thereby obtaining a biaxially oriented nylon 6film. The respective temperatures within the tenter were 60° C. forpreheating, 85° C. for drawing, 40° C. for the subsequent cooling, and225° C. for heat-setting. The film was then wound in the usual manner.In the present case, the ratio L/W was 1.0.

Example 10

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 9, except that in the present case the ratio L/W was 2.0.

Example 11

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 9, except that in the present case the ratio L/W was 3.0.

Comparative Example 5

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 9, except that in the present case there was no coolingprocess (i.e., L/W=0).

Table 2 shows the film-producing conditions in the above-mentionedExamples and Comparative Examples, a parameter of molecular orientationangle ΔΘ_(or) ×W/Wf of the obtained film and the size of curlingobserved when the films were formed into bags.

                  TABLE 2                                                         ______________________________________                                        Resin         Cooling         Δ θ                                                                        Size                                   used for      tempera-        or × W/W.sub.f                                                                   of curl-                               film          ture (°C.)                                                                      L/W    (°)                                                                             ing.sup.1)                             ______________________________________                                        Example                                                                              Polyethylene                                                                             40       1.0  52.0     N                                    5      terephthalate                                                          Example                                                                              Polyethylene                                                                             40       2.0  27.0     N                                    6      terephthalate                                                          Example                                                                              Polyethylene                                                                             40       3.0  22.9     N                                    7      terephthalate                                                          Example                                                                              Polyethylene                                                                             65       1.0  62.4     N-M                                  8      terephthalate                                                          Compar-                                                                              Polyethylene                                                                             --       --   83.2     L                                    ative  terephthalate                                                          Example                                                                       Example                                                                              Nylon 6    40       1.0  62.2     N-M                                  9                                                                             Example                                                                              Nylon 6    40       2.0  53.0     N                                    10                                                                            Example                                                                              Nylon 6    40       3.0  46.1     N                                    11                                                                            Compara-                                                                             Nylon 6    --       --   96.8     L                                    tive                                                                          Example                                                                       5                                                                             ______________________________________                                         .sup.1) N: No curling                                                         M: Middle size                                                                L: Large size                                                            

As clearly shown in Table 2, the thermoplastic resin films ofComparative Examples displayed large variations of molecular orientationangle in the transverse direction and a large degree of curling occurredwhen the films were formed into bags. On the other hand, thethermoplastic resin films of the present invention possessed uniformphysical properties in the transverse direction (only little variationin molecular orientation angle along the transverse direction) and onlyslight curling occurred when formed into bags.

Example 12

A nylon 6 resin was melted and extruded from a T-die, and after forminginto a film on a chill roll, drawn 3.25 times in the longitudinaldirection with a roll drawing machine. Then the film was drawn 3.5 timesin the transverse direction with a tenter, and finally subjected toheat-setting treatment, thereby obtaining a biaxially oriented nylon 6film. The respective temperatures within the tenter were 60° C. forpreheating, 85° C. for drawing, 40° C. for the subsequent cooling, and220° C. for heat-setting. After further heat treatment at 210° C., thefilm was cooled to 100° C., removed from clips and then wound in theusual manner. In the present case, the ratio L/W was 1.0.

Example 13

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 12, except that in the present case the ratio L/W was 2.0.

Example 14

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 12, except that in the present case the ratio L/W was 3.0.

Example 15

The biaxially oriented nylon 6 film obtained in Example 13 was furtherprovided to a pair of rolls at 195° C., allowing the film to shrink by5% in the longitudinal direction, thereby obtaining a biaxially orientednylon 6 film.

Example 16

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 12, except that in the present case the substantially undrawnfilm was immersed in water before drawing in the longitudinal direction.

Example 17

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 12, except that in the present case the uniaxially orientedfilm was immersed in water before drawing in the transverse direction.

Example 18

The biaxially oriented nylon 6 film obtained in Example 17 was furtherallowed to shrink by 8% in the longitudinal direction with rolls,thereby obtaining a biaxially oriented nylon 6 film.

Example 19

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 12, except that in the present case the temperature of thecooling zone was 80° C.

Example 20

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 14, except that in the present case the temperature of thecooling zone was 80° C.

Example 21

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 13, except that in the present case the temperature of thecooling zone was 80° C. and the biaxially oriented nylon 6 film soobtained was further allowed to shrink in the longitudinal directionwith rolls.

Example 22

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 18, except that in the present case the film was heat-treatedwith water vapor of 95° C. or more while shrinking the film in thelongitudinal direction with rolls.

Comparative Example 6

A biaxially oriented nylon 6 film was obtained by the same procedure asin Example 18, except that in the present case, neither the coolingprocess nor the shrinking process in the longitudinal direction wasemployed (i.e., L/W=0).

Table 3 shows the parameter ΔBS ΔΘ_(or) /Wf² relating to the boilingwater shrinkage distortion factor and molecular orientation angle of thefilms obtained in the above-mentioned Examples and Comparative Example,as well as the degree of printing misalignment observed after marks hadbeen printed onto the said films.

                                      TABLE 3                                     __________________________________________________________________________           Water content of     Shrinking                                                film (%)   Cooling conditions                                                                      ratio in                                                      Uniaxially                                                                          Tempera-  longitudinal                                             Undrawn                                                                            oriented                                                                            ture      direction                                                                           Δ BS · Δ θ                                         or W.sub.f .sup.2                                                                      Printing.sup.2)                           film film  (°C.)                                                                       L/W  (%)   (% · deg/m.sup.2)                                                             misalignment                       __________________________________________________________________________    Example 12                                                                           <1%  <1%   40   1.0  0%    20.8     N                                  Example 13                                                                           <1%  <1%   40   2.0  0%    14.6     N                                  Example 14                                                                           <1%  <1%   40   3.0  0%    11.0     N                                  Example 15                                                                           <1%  <1%   40   2.0  5%    7.6      N                                  Example 16                                                                           9.1% 1.6%  40   1.0  0%    18.4     N                                  Example 17                                                                           <1%  8.6%  40   1.0  0%    16.4     N                                  Example 18                                                                           <1%  8.6%  40   1.0  8%    10.2     N                                  Example 19                                                                           <1%  <1%   80   1.0  0%    44.0     A                                  Example 20                                                                           <1%  <1%   80   3.0  0%    37.6     N-A                                Example 21                                                                           <1%  <1%   80   2.0  6%    17.4     N                                  Example 22.sup.1)                                                                    <1%  8.6%  40   1.0  8%    9.2      N                                  Comparative                                                                          <1%  <1%   --   --   0%    48.4     S                                  Example 6                                                                     __________________________________________________________________________     .sup.1) The film obtained in Example 18 was heattreated with water vapor      while shrinking in the longitudinal direction.                                .sup.2) N: negligible, A: appreciable, S: substantial.                   

As clearly shown in Table 3, the thermoplastic resin films of thepresent invention exhibited small values of the parameter ΔBS ΔΘ_(or)/Wf², and after printing, almost no printing misalignment was observedon these films. By contrast, the thermoplastic resin films of theComparative Example displayed large values of ΔBS ΔΘ_(or) /W_(f) ² aswell as large printing misalignment, indicating nonuniform physicalproperties in the transverse direction.

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom the scope and spirit of this invention. Accordingly, it is notintended that the scope of the claims appended hereto be limited to thedescription as set forth herein, but rather that the claims be construedas encompassing all the features of patentable novelty that reside inthe present invention, including all features that would be treated asequivalents thereof by those skilled in the art to which this inventionpertains.

What is claimed is:
 1. A method for producing a thermoplastic polyamideresin film at least oriented in the transverse direction, wherein thedifference between the molecular orientation angles at any different twopoints on a straight line in the transverse direction measured bymicrowave techniques satisfies the following formula V:

    ΔΘ.sub.or ×w/wf≦64.0              (V)

wherein ΔΘ_(or) is the difference of the molecular orientation angles(°) measured at said two points, wf is the distance (m) of said points,and w is the width (m) of said film, which comprises the steps ofdrawing a thermoplastic polyamide resin film in the transverse directionin a drawing zone, said film containing at least 1.0% by weight of waterbased on the total weight of said film and water, cooling said film to atemperature lower than the glass transition temperature of saidpolyamide resin in a cooling zone, heat-setting said film in aheat-setting zone, and allowing said film to shrink by a positive amountin the longitudinal direction, wherein the length of said cooling zonesatisfies the following formula VI:

    (L/W)≧1.0                                           (VI)

wherein L is the length (m) of said cooling zone, and W is the width (m)of said film after the drawing is carried out.
 2. A method according toclaim 1, wherein said thermoplastic polyamide resin film is cooledwithout holding both sides of said film.
 3. A method according to claim1, wherein said thermoplastic polyamide resin film is passed through agroup of nip rolls with controllable speed either during said coolingprocess or after said heat-setting process, or both.
 4. A method forproducing a thermoplastic polyamide resin film at least oriented in thetransverse direction, which comprises the steps of drawing athermoplastic polyamide resin film in the transverse direction in adrawing zone, said film containing at least 1.0% by weight of waterbased on the total weight of said film and water, cooling said film to atemperature lower than the glass transition temperature of saidpolyamide resin in a cooling zone, heat-setting said film in aheat-setting zone, and allowing said film to shrink by a positive amountin the longitudinal direction,wherein said cooling zone is provided witha group of nip rolls, and the length of said cooling zone satisfies thefollowing formula VII:

    (L/W)≧0.25 (2.0-W.sub.N /W).sup.2                   (VII)

wherein L is the length (m) of said cooling zone, W is the width (m) ofsaid film, and W_(N) (m) is the width (m) of the widest nip roll in saidgroup of nip rolls.
 5. A method according to claim 1, wherein saiddrawing zone has at least two drawing sections, the temperatures of saiddrawing sections being set progressively higher toward the cooling zone;and wherein said heat-setting process comprises first and secondheat-setting processes and wherein said film shrinking step is conductedduring the second heat-setting process, said first heat-setting processbeing carried out at a temperature in the range of 50° C. higher thansaid glass transition temperature to 20° C. lower than said meltingtemperature, while shrinking said film by a positive amount of 10% orless in the transverse direction, and said second heat-setting processbeing carried out at a temperature in the range of 100° C. higher thansaid glass transition temperature to 20° C. lower than said meltingtemperature, while shrinking said film by 10% or less in thelongitudinal direction.
 6. The method according to claim 1 for producinga thermoplastic polyamide resin film at least oriented in the transversedirection, wherein the thermal shrinkage stress in the transversedirection of said film satisfies the following formula I, and thethermal shrinkage factor of said film in the transverse direction at atemperature that is 40° C. higher than the glass transition temperatureof said resin is 5% or less:

    (σ.sub.2 /σ.sub.1)≦1.0                  (I)

wherein σ₁ is the thermal shrinkage stress (kg/mm²) of said film in thetransverse direction at a temperature that is 70° C. higher than theglass transition temperature of said resin, and σ₂ is the thermalshrinkage stress (kg/mm²) of said film in the transverse direction at atemperature that is 80° C. lower than the melting temperature of saidresin.
 7. The method according to claim 1 for producing a thermoplasticpolyamide resin film at least oriented in the transverse direction,wherein the molecular orientation angle in a marginal portion of saidfilm, measured by microwave techniques, satisfies one of the followingformulae II, III, and IV:

    -90°<A≦-60°                           (II)

    -30°≦A≦30°                     (III)

    60°≦A≦90°                      (IV)

wherein A is a molecular orientation angle (°), said angle beingmeasured with the longitudinal axis corresponding to 0° and clockwiserotation being regarded as positive.
 8. A method for producing athermoplastic biaxially oriented polyamide resin film having alongitudinal axis, wherein the boiling water shrinkage distortionfactors (%) at any different two points on a straight line in thetransverse direction and the molecular orientation angles at anydifferent two points on a straight line in the transverse directionmeasured by microwave techniques satisfy the following formula VIII:

    ΔBS×ΔΘ.sub.or /Wf.sup.2 ≦44.0 (VIII)

wherein ΔBS is, in terms of absolute value, the difference between saidboiling water shrinkage distortion factors (%) measured at said twopoints; ΔΘ_(or) is, in terms of absolute value, the difference betweensaid molecular orientation angles measured at said two points, and Wf isthe distance (m) between the points of measurement of said molecularorientation angle, said boiling water shrinkage distortion factor ateach of said points being the value obtained by subtracting the boilingwater shrinkage factors (%) measured in the direction of -45° from thelongitudinal axis from the boiling water shrinkage factors (%) measuredin the direction of +45° from the longitudinal axis, wherein thelongitudinal axis of the film is taken as 0°, which comprises the stepsof drawing a thermoplastic polyamide resin film in the longitudinaldirection in a first drawing zone, drawing said film in the transversedirection in a second drawing zone, said film provided for thetransverse drawing containing at least 1.0% by weight of water based onthe total weight of said film and water, cooling said biaxially drawnfilm at a temperature lower than the glass transition temperature ofsaid resin in a cooling zone, heat-setting said film in a heat-settingzone, and allowing said film to shrink by a positive amount in thelongitudinal direction, wherein the length of said cooling zonesatisfies the following formula VI:

    (L/W)≧1.0                                           (VI)

wherein L is the length (m) of said cooling zone, and W is the width (m)of said film after the drawing in the transverse direction is carriedout.
 9. A method according to claim 8, wherein said thermoplasticpolyamide resin film provided for the first drawing zone is an undrawn,uniaxially or biaxially oriented polyamide film.
 10. The methodaccording to claim 8, wherein said film shrinking step is conductedduring the heat-setting step.
 11. The method according to claim 10,wherein said film shrinking step is conducted by blowing water vapor ata temperature of at least 95° C.